To maintain a desirable internal combustion engine air/fuel ratio, such as the stoichiometric ratio, the amount of fuel delivered to the engine is determined in response to an estimated or measured engine cylinder inlet air mass. Mass airflow sensors are available for measuring cylinder inlet air mass directly under steady state conditions characterized by substantially no intake manifold filling or depletion, but are not generally responsive enough to provide for accurate cylinder inlet air mass information under transient conditions characterized, for example, by significant time rate of change in engine intake manifold air pressure. Speed density approaches are sufficiently responsive to provide accurate cylinder inlet air mass information during even severe engine transient conditions, and therefore are known to be useful as a supplement to mass airflow sensor-based approaches during transient conditions. However, conventional speed density approaches suffer shortcomings in inlet air mass measurement accuracy under certain engine operating conditions. Inaccurate engine cylinder inlet air mass measurement can lead to deviations in engine air/fuel ratio away from a desired air/fuel ratio, such as the stoichiometric ratio, leading to increased engine emissions and reduced engine performance. It would be desirable to resolve the accuracy shortcomings in conventional speed density based engine air/fuel ratio control approaches.
The speed density approaches provide engine cylinder inlet air mass m as a function of engine intake manifold pressure MAP, for example using the ideal gas law, which may be expressed as EQU m=MAP * V * VE/(R * T)
in which V is cylinder volume, VE is volumetric efficiency, R is the ideal gas constant, and T is air temperature. While the ideal gas law includes an air temperature term, the volumetric efficiency term applied with the ideal gas law to determine engine inlet air mass is conventionally determined using static calibration parameters. While such engine parameters as engine valve timing and engine cylinder port geometry, on which VE depends, do not change substantially during engine operation, other parameters on which VE depends, such as engine cylinder combustion temperature, can change significantly during engine operation, resulting in substantial open-loop engine air/fuel ratio error. VE is typically calibrated at a calibration combustion temperature. However, when the actual combustion temperature varies away from such calibration temperature, the heating of the fuel and air entering cylinder causes a gas expansion not comprehended by the VE calibration, resulting in a significant variation in the actual engine air/fuel ratio away from a desirable ratio. It would be desirable to compensate for such effects that drive actual volumetric efficiency away from calibration values, to improve engine air/fuel ratio control accuracy.